The present invention relates generally to brushless direct-current motors not having a sensor to detect the position information of the rotor, and more particularly to such a brushless motor equipped with a system to adequately and effectively perform the drive current phase control from the start timing.
Generally, in brushless direct-current (DC) motors to be used in hard disk drives (HDD), video tape recorders (VTR) and the like, the relative position between the magnetic pole and the drive coil is arranged to be detected by means of a position sensor such as a Hall device so as to control the phase of the current to be supplied to the drive coil to obtain a desirable rotation of the motor. On the other hand, brushless DC motors which are arranged to perform the drive current phase control without using such a position sensor (which will be referred hereinafter to as sensorless type brushless DC motors) have recently been developed for the apparatus size reduction and cost reduction purposes. An important problem arising with such sensorless, brushless DC motors relates to the fact that the relative position relation between the magnetic pole and the drive coil is not constant at the time of the start of rotation of the brushless motor, whereby difficulty is encountered to adequately and effectively perform the drive current phase control from the time of the rotation start. One known technique to eliminate this problem is disclosed in the U.S. Pat. No. 4,876,491. This technique involves performing the comparison between the currents to the multi-phase coils to detect the position of the rotor on the basis of the comparison results obtained from a train of current pulses. However, because of the relative position detection being based on the current detection, this technique requires series resistors in the drive circuit whereby there is the possibility that the electric power loss occurs on the normal rotation.